Free radicals and reactive oxygen species readily interact with polyunsaturated fatty acids in cell membranes resulting in the formation of cytotoxic aldehydes. These aldehydes are considerably more stable than free radicals and thus are likely to diffuse into the cellular medium. Among these aldehydes, 4-hydroxy-2-nonenal (HNE) is one of the major products of membrane peroxidation and shows many biological effects. Lipid peroxidation increases during aging and in many diseases, including atherosclerosis. Modification of proteins and other biomolecules by lipid peroxidation products is therefore believed to have a significant role in senescence and in some diseases. The mechanisms by which HNE acts are not, however, well understood. Human Aortic Endothelial Cell (HAEC) was chosen as a model for studying the effects of HNE on human endothelium. Incubation of HAEC with HNE resulted in an increase in the level of protein carbonyl groups. The formation of HNE-protein adduct in the cell upon the treatment of HNE is presently under investigation. The results of this and related experiments may provide better understanding on the role of lipid peroxidation in disease- and age-related accumulation of damaged protein and impairment of cellular function.